Many semiconductor imaging sensors today are front side illuminated. That is, these sensors include imaging arrays that are fabricated on the front side of a semiconductor wafer, where incoming light is received at the imaging array from the same front side. Front side illuminated imaging sensors have several drawbacks, for example, a limited fill factor.
BSI imaging sensors are an alternative to front side illuminated imaging sensors. BSI imaging sensors include imaging arrays that are fabricated on the front surface of the semiconductor wafer, but receive incoming light through a back surface of the wafer. BSI imaging sensors may be formed by binding a device wafer to a carrier wafer, followed by thinning down the device wafer. To permit detection of external incoming light from the backside, the device wafer is made extremely thin. For example, the final device wafer thickness in some BSI imaging sensors is only several microns.
Light that is not external incoming light may be emitted within the silicon substrate of the device wafer by peripheral circuit elements. This internally generated light may be optically channeled and travel laterally within the silicon substrate of the device wafer to reach imaging arrays. Such lateral light propagation may produce undesirable signals and interfere with the normal operation of BSI imaging sensors. The relative thinness of the silicon substrate may contribute to lateral light propagation.